It is well-known to use these fuels in admixture with the intake air. Patent application WO 96/35,863 describes a system based on this concept where air and liquefied fuel are mixed upstream from the combustion chambers as a complement to a conventional gasoline injection system (of "common rail" type here). However, fuel vaporization problems are observed with this type of supply. To overcome this problem, this prior art proposes mixing the liquid fuel with the intake air in a place relatively distant from the combustion chamber; the temperatures are thus less high and better vaporization can be performed.
Recent developments have allowed to introduce the fuel in liquid form, directly or mixed with air, into the combustion chamber. This requires pressure maintenance in all the supply system, from the tank to the vaporization point: combustion chamber in the case of direct injection or intake pipes for indirect injection.
An important problem then consists in maintaining and controlling the liquid state by means of the pressure in all of the supply system, whatever the running conditions of the engine and the outside conditions. In fact, the liquefaction pressure variation can range from 1 to 18 bars according to the working temperature of the fuel.
The fuel tank is generally designed to withstand this type of pressures so that storage in the liquid state does not pose problems at this level of the supply circuit.
However, this is not the case in the other parts of the supply circuit and notably between the tank and the injectors.
More precisely, a problem arises when the engine is restarted after prolonged stoppage of the engine.
This phenomenon can be explained as follows: the injectors being permeable to the fluid, a slight liquid leak occurs when the engine is stopped, which lowers the pressure in the supply system and therefore changes the state of the fuel that goes from the liquid to the gaseous state. The leak is thus sustained.
After a more or less long period of immobilization of the vehicle, which depends on the storage conditions and on the structure of the injectors, the (originally liquid) fuel contained in the supply system can thus be vaporized and transferred to the engine, either directly into the combustion chamber or via the intake.
When the engine is restarted, the vaporized fuel is present in large amounts in the combustion chamber, which generates a non-flammable mixture (case of liquefied petroleum gas LPG) or self-ignition (case of dimethylether DME).
A known solution consists in isolating each injector of the rail by a solenoid valve when the vehicle is stopped. However, this solution is complex, bulky and expensive.